Reduction of explosive shock and noise by dispersion of water particles



I r I .7 m FIwgll W 369797563: CWUW mm Aug. 20, 1968 A. B. ANDREWS ET AL 3,3

REDUCTION OF EXPLOSIVE SHOCK AND NOISE BY DISPERSION OF WATER PARTICLES Filed July 29, 1965 2 Sheets-Sheet 1 A 8 I2 mm Q FIG. 2

ZNVENTOR ALDAY BISHOP ANDREWS VID LINN COURSEN ATTO JEY Aug.20, 1968 A. B. ANDREWS ET AL 3,397,756

REDUCTION OF EXFLOSIVE SHOCK AND NOISE BY DISPERSION OF WATER PARTICLES Filed July 29, 1965 2 Sheets-Sheet 2 FIG6 MINIMUM CHAMBER DIAMETER VS. CHARGE WEIGHT GRANITE GHEISS MINIMUM CHAMBER DIAMETER FT.

2 5 4 56T89I00 2 3 456789I000 2 3 4 6 8 CHARGE MEIGHT- POUNDS INVENTORS ALDAY BISHOP ANDREWS DAVID LINN COURSEN ATTORNEY United States Patent 3,397,756 REDUCTION OF EXPLOSIVE SHOCK AND NOISE BY DISPERSION OF WATER PARTICLES Alday B. Andrews, Woodbury, N.J., and David L'. Coursen, Newark, Del., assig-nors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed July 29, 1965, Ser. No. 475,719 11 Claims. (Cl. 18133) This invention relates to a process for reducing noise generated by explosions and to a novel, reusable structure "for mufiling such noise.

Much attention has been given to the control of noise, especially in more or less densely populated areas where noise created by construction, manufacturing and t-rans portation operations easily reaches the annoyance level, and may even represent a health hazard. Sound and vibration absorbing devices have been designed for the mounting of heavy reciprocating and rotating machinery, and mufflers have been designed for exhausts of all types including air and steam jets and for the exhausts from internal combustion engines of both spark and diesel types. Likewise, considerable attention has been given to control of the exhaust noises associated with testing of jet propulsion devices. The use of mufiilers and silencers on firearms also is well known. Each of these noise sources has characteristics that set it apart from the others and requires more or less novel means for controlling and limiting the associated noise.

The prior art also discloses noise-reducing structures which attenuate noise created by detonation of an explosive contained therein. Such structures are enclosures which can be reused a great many times. Said structures not only confine missiles but also reduce the noise created by detonations to a level which is not objectionable in the space outside said structure.

A distinguishing characteristic of detonation as a noise source is the exceedingly high rate of reaction and the almost instantaneous generation of a sharp shock front and intense sound pulses, as compared with defiagration reactions which, relatively, are much slower combustion or burning processes. In detonation the noise source is at the point of detonation, whereas deflagr'ations inherently are low in noise level until the pressure built up over a relatively long reaction period by combustion reactions suddenly is released, as by deliberate venting or by acciden al rupture of the container. with the generation of a sharp intense sound pulse, perhaps 'at an objectionable or hazardous level. The differences in time and intensity factors make more difficult the control and mufiiing of noise generated by detonations as compared with deflagrations, which latter include explosions in internal combustion engines. In general, structures that 'are effective in mufiling noise generated by detonations also are effective in controlling such noise as is generated by pressure release after deflagration of compositions of equal total energy content; but conventional structures suitable for controlling forces generated by a deflagration may be disastrously ruptured by detonation of a composition of equal total energy content because of the intensity of the shock wave and the almost instantaneous high pressure generated by a detonation. This distinction is well recognized, for example, in firearms, many of which would be ruptured should detonation, instead of deflagration, of the propellant charge occur.

An especially effective noise-reducing structure for use with detonating explosives is described in United States Patent No. 2,940,300. This patent also discloses principles of design for such a noise-reducing structure whereby said structure will withstand the disruptive force created by 3,397,756 Patented Aug. 20, 1968 repeated explosions of masses of detonating materials not greater than the maximum established by the design relationships disclosed in said patent.

An improvement in the noise-reducing structure of United States Patent No. 2,940,300 is disclosed in US. Patent No. 3,165,916 wherein the noise-reducing structure contains an internally disposed mass of particulate material, the presence of which permits detonating a significantly increased amount of explosive within the described structure without exceeding the strain permitted by the design criteria, and without loss of the noise-reducing feature of said structure.

In both of the aforementioned noise-reducing structures the energy released by the detonation, including the sound energy, is absorbed or attenuated almost completely by the structures and the corltents thereof, except for that portion of energy which is dissipated by the passage of reaction gases through the gas-venting means, said gasventing means being a critical feature of such structures. Gas-venting means employed in structures of the prior art preferably are fixed, gas-type mufilers which provide 'a tortuous path for the explosion gases as they pass out of the structure to the space outside.

Noise reducing structures of the type described above for use with detonating explosives are both practical and effective in mufiiing noise created in small-scale testing work, for example with less than pounds of detonating explosive, and in explosion-powered metalworking operations carried outwith similar quantities of explosives. Indeed, the principles disclosed for design of said noise-reducing structures also are applicable for much larger structures which, however, required very massive pressu-reand shock-resistant steel walls which are both difiicult and costly to construct, operate, and maintain.

A marked increase in the use of detonating explosives for metal-working operations, especially for the explosion cladding of metals, as described in United States Patent No. 3,137,937 has created a need for noise-controlling structures which will permit the manufacture in or near populated areas of even larger clad plates than heretofore available, plates the size of which is established and limited primarily by the capacity of equipment manufacturers to handle and,to fabricate such plates. Generally speaking, heretofore the capability of equipment fabricators to handle and convert metal plates into reactors, tanks, and other similar process equipment has far exceeded the ability of metal suppliers to manufacture correspondingly large plates by explosion-cladding processes. As a result, a demand has existed for large sized clad metal plates which best can be made by the explosioncladding process disclosed in aforementioned US. Patent No. 3,137,937 and in many cases can be made only by this process.

The cladding of large metal plates entails the use of relatively large quantities of detonating explosive. Detonation of charges of hundreds or even thousands of pounds of high explosive on a repetitive basis for the cladding of large metal plates by the aforementioned explosioncladding process may be carried out in the open in a sparsely populated, isolated location where the shock and noise created} by said detonations is not objectionable. As a practical matter, however, the transport of men, materials, and equipment to and from such a suitably isolated area is impracticable because it is time consuming and excessively costly. Thus, it is apparent that the need for quantities of massive metal plates of clad construction can be satisfied practically and economically only by use of a suitable noise-controlling, explosion-cladding facility located near both the sources of supply of the metal plates to be clad and the mechanical shops where said plates can be prepared for cladding and for processing after cladding. Preferably, the cladding facility also will be reasonably close to the shops in which the clad plates are to be fabricated into equipment.

This invention provides a simple, effective and inexpensive means for reducing the shock and noise caused by detonation of explosives. The structures of this invention can repeatedly mufile noise and shock of large masses of explosives without significant damage thereto.

The process of this invention is a process for reducing the shock and noise caused by detonation of a main explosive charge which comprises interposing a body of finely divided particles of water in the path of the shock wave from said main charge, said water being propelled into said path by a dispersing explosive charge spaced apart from the main charge.

The noise-reducing structure provided in accordance with this invention, and in which the process of this invention preferably is practised, comprises a stable subterranean chamber, a tunnel connecting said chamber to a portal, at least one body of water disposed in said tunnel, and at least one explosive charge positioned to disperse said water in said tunnel. Preferably, the portal of the tunnel is provided with a massive movable closure, preferably having a mass of at least about 100 tons.

In its broadest aspects, the process of this invention can be used to shield any detonation and is not limited to subterranean operations. Thus, for example, in an open area, a charge, the detonation of which is to be shielded, can be surrounded with a quiescent annulus of water with at least one explosive charge positioned therein. Detonating the dispersing charge in the water annulus creates an annular body of dispersed water particles that dissipates, modifies and reduces the harmful and destructive effect of the shock wave passing therethrough. Preferably, however, the process is carried out in a subterranean cavity wherein the explosively generated water dispersion modifies the shock wave emanating from the cavity, preferably to reduce the sound level to below 130 decibels at any position 400 feet or more from the cavity portal. In general, the amount of water which must be dispersed in the path of the shock wave to be muffled increases with the amount of mufiling desired and with that part of the emanating shock wave which is to be blocked by the water dispersion as contrasted with other means, e.g., in subterranean shooting, by overburden. The minimum amount of water is that necessary to effect the muffiing desired while, as a practical matter, the maximum is that for which the charge required for dispersal causes a shock no greater than the muffled shock wave from the main detonation. For subterranean shooting in a tunnel or chamber where the shock wave emanating from a portal is to be blocked, generally the amount of water dispersed is about from to 100 times the weight of explosive detonated in the main charge.

As previously indicated, the water is explosively dispersed by a dispersing charge. The minimum charge is that necessary to get the dispersion, hence the muffling, desired while, for practical purposes, the maximum is that which creates a shock wave no greater than the muffled main shock wave. Usually, a dispersing charge of one pound of explosive for about from 100 to 15,000 pounds of water is used. A preferred amount is about one pound of an explosive for each 1500 pounds of water. The dispersed water preferably should occupy a substantial volume and usually be disposed at least 10 .feet along the path of the main shock wave. Also, for optimum dispersion, the dispersing explosive charge should be distributed through the body of water. In subterranean shooting it is practicularly convenient to lay an elongated body, e.g., a trough or large easily fractured tube of water along one or both sides of the tunnel leading to the portal. Preferably, the dispersed water is positioned apart from the main explosion, for example, to 200 feet therefrom depending on the type of shooting involved. The dispersing explosion is timed to occur shortly prior to passage of the shock wave generated by detonation of the main charge so that maximum dispersion of water is achieved at the time of passage of said main shock wave.

In the event that noise from a dispersing charge reaches an objectionable level, said noise similarly may be muffied by forming a second and smaller mass of dispersed water positioned between said first mass and the observation point. In the case in which more than one dispersing charge is employed, it is usually preferred that the dispersing charge furthest from the main charge create a shock Wave no greater in intensity than that of the shock wave from the preceding charges which it mufiles, i.e., the shock wave from the main charge and any preceding dispersing charges.

In an especially preferred noise-reducing process and structure of the present invention effective control of noise levels within acceptable limits is obtained by retaining the detonation-generated sound within the sub terranean cavity for at least 25 milliseconds, that is, until the shock wave energy is dissipated to such an extent that the residual sound released from said structure is at an unobjectionable level of intensity. This will correspond to a I'etention time greater than the time required for both the initial direct shock wave from detonation and the wave reflected from the rear of the chamber to reach the portal.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings, of which:

FIGURE 1 is a view in sectional elevation of a subterranean noise-reducing structure of the present invention;

FIGURE 2 is a plan view of a subterranean noise reducing structure of FIGURE 1, and FIGURE 2A is an enlarged cross-sectional elevation of the tunnel portion of the structure of FIGURES l and 2 at position A-A.

FIGURE 3 is an enlarged side view of a movable plug closure as shown in FIGURE 1;

FIGURE 4 is an end view of the movable plug closure shown in FIGURES l and 3.

FIGURE 5 is an enlarged plan view of a movable plug supported on two railroad gondola cars; and

FIGURE 6 is a graph relating chamber diameter to mass of explosive to be detonated therein, as described hereinbelow.

In FIGURES l to 5, like numbers refer to like parts. Referring now more particularly to FIGURE 1, which is a cross-sectional elevation of one embodiment of a structure of the present invention and certain appurtenances thereto, 1 represents a substantially homogeneous natural rock mass, for example, a granite gneiss; 2 is a stabilized subterranean chamber, as defined herein, excavated from and surrounded by rock mass 1; 3 is a mass of sand placed on the fioor of chamber 2 substantially in the center of the floor area; 4 is a metal plate resting on said mass 3 in a position ready for cladding; 5 is a tunnel connecting chamber 2 to portal 7 which provides entry to tunnel 5; 6 is a trench in the floor of tunnel 5, designed to be filled with water, and positioned along the wall of tunnel 5 inside portal 7, which is fitted with rib-reinforced steel arch and liner 8 which, in turn, is anchored in place by steel roof bolts 9 and cement grout fill (not shown) between ribbed arch 8 and the wall of tunnel 5; 10 is the lip of arch 8; 11 is a movable plug closure for portal 7, said closure plug being terminated at the tunnel end by gasketed mating surface of tongue 12 of such size and shape as to permit engaging the inner face of lip 10 of arched-rib structure 8; 13 is a steel gondola car mounted on standard gage railroad trucks 14, said trucks 14 being fitted with standard railroad air brakes (not shown), and said car and trucks carrying at one end, and attached thereto, movable closure plug 11, said car containing a ballast mass 15, and resting on steel rails 16; and 17 is a pile of granular material, for example sand, of sufiicient size and mass to arrest the movement of car 13 and plug 11 mounted thereon when plug 11 is propelled from its closure position in contact with lip of arch 8 surrounding the sides and top of portal 7.

FIGURE 2 is a plan view of a noise-reducing structure of the present invention and certain appurtenances thereto wherein the numbered parts have the same significance as in FIGURE 1 above.

FIGURE 3 is an enlarged side view of movable plug 11 mounted on gondola cars 13 and rail trucks 14, wherein flange 18 is shown between the body of movable plug 11 and its gasketed tongue 12.

FIGURE 4 is an end view of plug 11 showing flange 18, gasketed tongue 12, supporting and reinforcing structural steel members 19, and the ends of rails 16 which support said cars and movable plug assembly.

FIGURE 5 is an enlarged plan view of the assembly of movable plug 11 and gondola cars 13 mounted on rail trucks 14.

It is obvious that the subterranean chamber, e.g., 2 in FIGURE 1, of the noise-reducing structure of the present invention must retain its structural size and shape after repeated detonation of explosives therein. If the walls, floor, and ceiling are damaged or portions thereof are loosened or dislodged by one or more blasts, the chamber in an extreme case may be subject to catastrophic collapse, or at least it may become hazardous for further use, for example, due to the danger of rock falls on personnel, on equipment, and on the explosive charges or assemblies placed within said chamber. That is to say, the chamber must retain its structural integrity throughout a period of continued use. Such a chamber is designated herein as a stable subterranean chamber. Obviously, the stability of the chamber will be related to the physical characteristics of the material surrounding the chamber, to the properties of the explosive charge to be detonated therein, to the size of the chamber with respect to the energy of the detonating explosive, and to the placement of the explosive charge with respect to the walls of the chamber.

Traditionally, the purpose of detonating an explosive charge in a borehole in rock is to break the rock and dislodge a portion of rock from the massive rock structure. It is well known that a detonating explosive charge which does not fill the borehole is less effective for breaking rockthan is a charge of the same weight that completely fills its borehole. Detonatin g explosive charges in cavities larger than the charge itself is known as decoupling the charge, and the decoupling ratio is defined for spherical charges as the ratio of the radius of the chamber to the radius of the charge. If decoupling is adequate, the elastic limit of the rock surrounding a detonating charge is not exceeded and fracture does not occur. The failure strength of SOme rocks was investigated by the US. Bureau of Mines (R.I. No. 6333, by T. C. Atchison, W. I. Duvall, and J. M. Pugliese) who established for cylindrical detonating charges in linear cavities a relationship between the failure strength of the rock and the critical decoupling ratio, i.e., at a lower than critical decoupling ratio fracture occurs and at a higher than critical decoupling ratio the integrity of the rock is maintained during and after a detonation of given intensity. For the explosion of high-velocity gelatin dynamite at a density of 1.4, the critical decoupling ratio for granite is about 4.2 and for limestone it is about 8 when the critical stresses for granite and limestone, respectively, are about 6400 and 500 p.s.i. Critical stresses as used herein are stresses produced by explosiomgenerated pressure pulses above which non-elastic effects including plastic flow, crushing and cracking of rock occur. For explosiongenerated pulses which produce less than critical stresses, the pressure pulse propagates through the medium without fracturing it, the pressure pulse being absorbed and dispersed in the surrounding medium. The decoupling ratio for a spherical charge in a spherical cavity can be estimated from the equation:

R p C D V37 RQ 401 wherein D is the decoupling ratio, R is the cavity radius, R is the charge radius, p is the density of the rock, o is the velocity of sound in the rock, D is the detonation velocity of the explosive, a is the tensile failure stress of the rock, and 'y is the ratio of specific heat of the detonation products at constant pressure to that at constant volume. By assuming that all the explosive in the charge is disposed in a hemisphere on a bed of crushed stone in the center of the floor of the cavity or chamber, and ob taining 0,, D, 0-, and 'y from standard references such as aforementioned Bureau of Mines publication R1. 6333, R can be calculated. The minimum distance to any wall of the cavity for the given charge is taken as 1.26 R to account for the hemispherical spreading of the blast wave. The results of these calculations are shown graphically in attached FIGURE 6 wherein the vertical, or y, axis represents the minimum diameter of the firing chamber in feet, and the horizontal, or x, axis shows the corre sponding charge weight in pounds. The solid lines 1 and 2 show this relationship for charges of high velocity dynamite in granite and in limestone, respectively. Thus, for any given charge weight of dynamite, or any other deto nating high explosive, a chamber in limestone must be of greater radius than for the same charge in a chamber in granite because of the lower critical stress value, that is the lower strength, of limestone. Even though the values for critical decoupling ratios and for critical stresses of granite and limestone arenot highly precise, experience has shown that subterranean chambers sized as described above are indeed stable subterranean chambers for noise-= reducing structures of the present invention.

The stabilized chamber in a structure of the present invention need not be spherical in shape. If the mass of explosive is distributed over a planar area, as exemplified in the process of US. Patent 3,137,937, said chamber 2 of FIGURE 1 will be a room having a substantially rectangular floor of length and width proportional to the areal dimensions of the largest explosive charge to be detonated therein, and a domed ceiling of such height as to exceed the design radius determined in the manner described hereinbefore. Although the size of the chamber or cavity also varies with the particular explosive used, usually it falls Within 20% of that calculated for dynamite as illustrated in FIGURE 6, being lower for low velocity, lower weight-strength, less brisant explosives and higher for high-velocity brisant explosives such as torpex.

The mass of granite rock surrounding said subterranean chamber generally should be at least three times as thick as the chamber diameter, and for limestone about 1% times as thick, for the largest masses of explosive to be detonated in said chamber of a noise-reducing structure of this invention. The mass of rock may be much thicker, and in addition usually is overlayed with a burden of rubble, soil and vegetation.

In carrying out the process of U8. Patent 3,137,937, for example, in a noise-reducing structure of the present invention, the metal-cladding assembly with explosive charge in place is supported on a bed of sand 3 in FIG- URES 1 and 2, about 12" thick and approximately in the center of the floor area of chamber I. Said sand, when smoothly leveled, provides a uniform support for metal plate-cladding assemblies placed thereon, and also protects the chamber floor from damage due to repeated blasts. The center of mass of the explosive charge will be no closer to the chamber side wall and ceiling than the design cavity radius determined as described hereir1- before. It will be apparent that the design principles which govern the shape and size of a structure of the present invention also establish the maximum explosive charge which permissibly is detonated within said structure,

All charges of lesser energy content likewise can be detonated Within said noise reducing structure without damaging the structure or exceeding the permissible noise limits.

The second element in the preferred structure of the present invention is a passageway or tunnel leading from said subterranean chamber 2 to the entrance portal 7 in attached FIGURE 1. The minimum length of the tunnel is established by the requirements for chamber overburden, i.e., the length of said tunnel must be great enough to provide at least the minimum thickness specitied hereinabove for the mass of rock which surrounds said subterranean chamber. Preferably, the tunnel Will be of greater than minimum length. Practically, the length of the tunnel will be governed by the topography of the area and will be limited by the high cost of driving excessively long tunnels and providing service facilities thereto. The tunnel is of such width and height as to permit easy transport of materials into chamber 2, but the width and height of said tunnel generally will be less than the width and height of said chamber. For use with 'rnaximum explosive charges, an irregular surface of said tunnel wall is preferred. The irregularities serve to attenuate the shock waves, especially those which are reflected from the face of plug 11 back to the rear chamber wall, and are then re-refiected to portal 7. Such wall roughness need not exceed of the tunnel diameter. The tunnel floor generally will be approximately level and on grade with the floor of chamber 2, and may be equipped with rails for rail transport of materials, may contain electric power service lines, firing lines, instrument lines, ventilation ducts and the like, none of which are shown in the appended figures.

Water is preferably dispersed in tunnel 5. In a particularly preferred embodiment of the present invention trenches 6 are positioned longitudinally in the floor of tunnel 5, preferably at the sides thereof, as illustrated in FIGURES 2 and 2A. Said trenches serve as containers for water which is explosively dispersed substantially coincidentally with the detonation of the mass of explosive in chamber 2 of a structure of this invention. Typically, for example, the trenches will have a capacity of about 3000 gallons of water for large blasts, say 1200 pounds of high explosive, in a structure of this invention. Less water will be required for smaller amounts of explosive. The water is dispersed within a segment of the tunnel volume by the detonation of explosive cord, or, alternately small charges spaced along the water to be dispersed, e.g., every 3 or 4 feet. The amount of explosive charge, based on the explosive therein, is within the ranges previously indicated.

As an effective alternate, water may be contained in flexible plastic tubes which rest on the floor over suitable lengths of detonating cord which will rupture said containers and disperse the water contained in said plastic bags. Other containers such as long troughs may be used, but may be subject to mechanical damage from blast effects. Mechanical means of providing a massive spray generally provide inadequate dispersion of the large amount of water in the short interval of time most suitable for noise control in the structure and by the method disclosed herein.

Tunnel 5 of FIGURES 1 and 2 and 2A terminates in portal 7. This portal may be cut in the solid massive rock if said rock is unfractured. If the rock surrounding the portal is not homogeneous and sound, the portal is reinforced by a steel arch or lining 8 bolted and grouted into place as shown in FIGURE 1. The portal, or steel liner thereof, will have as its outer periphery a lip 10 extending from top to floor level, said lip mating with the tongue 12 of movable plug closure means 11 when said plug is moved into closure position against portal 7.

A preferred element in a noise-reducing structure of this invention is a closure means for the tunnel. Closure may be achieved by mounting a blast door across the inner end of the tunnel and supporting it by the chamber walls, or by a blast door positioned across the tunnel at a point intermediate the chamber and the portal, and by providing a suitable auxiliary passageway for exit of the gases generated by explosion of the metal-cladding explosive charge in chamber 2. Because of the difficulty associated with mounting and use of the very massive blast door which is required to withstand the forces generated by explosion of large charges, a preferred clousure means is a movable plug 11 in FIGURE 1 which also is shown in more detail in FIGURES 1, 2, 3, 4, and 5, wherein the numbers correspond to the parts identified hereinbefore.

In use, after placing the explosive charge in chamber 2 of FIGURE 1, plug 11 is moved into place so that the gasketed surface of tongue 12 makes firm contact with lip 10 of arch 8, the brakes on the railway trucks 14 are firmly set by pneumatic pressure, or hydraulic pressure, applied by a conventional brake cylinder. Any small opening remaining at the bottom of the portal below the movable plug 11 is closed by banking plastic bags of water, earth, sand, or like particulate material in the free space below said plug. Closure of the portal area must be at least complete, and preferably will be greater than 97% complete. The explosive charge 4 is fired conventionally by use of electric blasting caps or by detonating fuse initiated from outside the plugged tunnel. Pressures generated by the explosion eventually push movable plug 11 away from the portal and release the attenuated pressure pulse at an unobjectionable sound level. For large blasts a sand pile 17 over tracks 16 in FIGURES 1 and 2 provides additional braking to limit the movement of closure plug 11. Forced circulation of ventilating air then frees the chamber and tunnel of fumes generated by detonation of said explosive charge, after which persons may reenter the tunnel and chamber.

Acoustical studies have shown, and experience in the use of noise-reducing structures of the present invention has confirmed, that few complaints are received from residents when explosion-generated sound levels from single blasts are below about decibels (reference pressure=0.0002 microbars) in the occupied areas. An adequate degree of control of noise, i.e., unwanted sound, created by detonation of explosives is achieved by a structure of the present invention if the sound created by a detonation within said structure is so modified and so attenuated that upon release and propagation from the portal of said structure, the sound is not objectionable (i.e., does not exceed about 110 db) in the inhabited areas nearest said structure. Thus, for example, a noise-reducing structure of the instant invention must be much more effective for a given weight of detonating explosive if the inhabited area is 1,000 feet from said structure than if it is 10,000 feet from said structure. In addition to distance, it is well known that factors related to weather conditions such as wind direction, velocity and temperature variations with altitude, humidity, and topography of the immediate area also affect the intensity of blast waves transmitted to the inhabited sensing area. For practical purposes of noise muffling in a specific situation, therefore, it is necessary only to know that a given level of noise in the immediate vicinity of a structure of this invention will not be objectionable after transmission, under the most favorable environmental circumstances therefore, to the nearest inhabited area. Hence, it is possible to carry out a large number of demonstrations of the effectiveness of the sound-reducing structure of this invention, and to monitor the effectiveness of said structure in regular use, by making sound measurements in the immediate vicinity of said structure, even though the noise level at the point of measurement appreciably exceeds the annoyance level of about 110 decibels at a more distant inhabited area.

Although the process and structure of this invention are particularly illustrated with respect to explosion clad- 9 ding, they are equally well suited for any application in which it is desired to mufiie the detonation of an explosive. Other examples of such applications are explosion forming, hardening and other metal working operations, as well as explosive testing. In addition" to effectively muffiing shock and sound, the process of this invention also advantageously modifies the pressure profil'e of the blast wave in that it is particularly effective irr" removing high frequency sound leaving low frequency components of the blast wave which are much less objectionable.

In the following examples, the effectiveness of a soundreducing structure of the present invention is demonstrated comparing the sound levels produced; by given .explosive charges fired in the open, in a stabilized subterranean chamber, and in accordance with this invention;

Example I 'A tunnel 300 ft. long is driven into a ridge of granite gneiss. The first 50 ft. of the tunnel has an 8-ft. high x 8-ft. wide cross-section, the next 100 ft. an 8-ft. high x l-ft. wide cross-section, the next 100 ft. similar to the initial section, and the final section, the stabilized firing chamber, has a 16-ft. high x -ft. wide cross-section and a domed ceiling. The trenches 6 on both sides of the tunnel floor along the first 100 feet from portal 7 are filled with water and together hold about 3000 gallons.

The explosive system inside the portal 7 comprises ,an electric blasting cap, a SO-ft. length of 2 gr../ft. LEDC (i.e., 2 gr./ft. PETN/ft. of low energy detonating cord- LEDC), an LEDC cross tie connected to two 400 git/ ft.

Primacord lines, each 100 feet long, one each being submerged in the water-filled trenches, and another LEDC cross tie from the end of the Primacord lines to a l-ft. section of 2 gr./ft. LEDC. This line is connected to" a 200-ms. (millisecond) conventional delay, which in turn is connected to a detonator in the main charge of 80/20 amatol composition placed in the center of the firing chamber on a bed of sand. An application of current to the electric blasing cap fires the cap which initiates the LEDC and the Primacord, detonation of which disperses the water, and then ignites the LEDC line leading to the main charge. The delay unit in the LEDC allows the water to become fully dispersed before the main explosive charge in chamber 2 is fired.

Several additional charges of cladding explosive are fired in the noise-controlling structure described in this example. The weights of charge, amount and kind of Primacord, and the sound level at the reference point 400 feet from portal 7 are shown in the following tabulation. For these tests, the time delay between detonation of said Primacord and the charge of cladding explosive is about 200 milliseconds to insure proper dispersal of water from trenches 6.

Primacord" (gr./ft.) Explosive (1bs.) Sound level at Measuring Point ((11).)

Reference experimentexplosive detonated without use of Primacord.

Analysis of sound profiles inside. and outside the portal, by use of pressure gages and a high speed oscillograph, shows that high frequency components of the blast wave are removed by passage through the dispersed water and that blast-wave pressure is reduced, for example, in the case of 120 lb. of explosive and 400 gr./ft. Primacord from 8.6 to 1.2 psi. After emission fromthe portal, the shock wave is absent and low frequency portions are substantially absent from the pressure profile because of the low radiation efficiency of the portal at low frequencies. In the absence of said-water spray (see table above), the shock wave from a 120 lb. shot has a peak amplitude 10 30 times (or 30 db) greater than the wave emitted after passing the shock wave through said water spray.

Example 2 In this example, a 1250 pound charge of -20 amatol explosive is detonated within a structure of'this invention comprising a 300 ft. tunnel in a ridge of granite gneiss. The first 250 ft. has a l2-ft. high x l2-ft. wide cross-section and the final 50 ft. has 14-ft. high x 18-ft. wide crosssection and a domed ceiling. The tunnel has a wall roughness of about 10% of its width. Water for the water spray is contained in two pairs of 16 in. diameter polyethylene Example 3 This example further illustrates the cumulative effects achieved by combination of the several elements of the noise-reducing structure of the present invention. The chamber and tunnel are as in Example 2 above, the movable plug closure 11 is as described hereinbefore and shown in the drawings, and the means foriproviding the massive water dispersion are as in Example 2 above. A 500 lb. charge of amatol explosive is fired in each test, the sound measurement being made at the reference point 400 feet from tunnel portal 7. The results of the'sound measurements for each of the test firings are shown in the following table:

Condition Sound level at Reference Point,

db. (ref .0002 ,ll bar) The examples hereinabove demonstrate the effectiveness of. the process and noise-reducing structure of the present invention in controlling explosion-generated noise, and holding it at levels acceptable to inhabitants in an area adjacent to said structure, for detonations of up to 1250 pounds. This, however, should not be regarded as a limitation on the size and capabilities of sound-reducing structures designed in accordance with the disclosures of this invention. The invention, therefore, is not limited to the exact details shown and described, but includes also various modifications which may appear to those skilled in the art, but which do not materially change the basic character of the invention or depart from the principle or spirit thereof. It also will be apparent that the process and structure of the present invention may be used in the conduct of other explosion-activated operations and with high explosives other than the explosives described herein.

We claim:

1. A process for reducing the shock and noise caused by detonation of a main explosive charge which comprises interposing a body of dispersed water particles in the path of the shock wave from said main explosive charge, said water being propelled into said path by a dispersing explosive charge spaced apart from the main charge.

2. A process for reducing the shock and noise emitted from the portal of a subterranean cavity and caused by detonation of a main explosive charge therein which comprises interposing a body of dispersed water particles in the path of the shock Wave fromsaid main explosive charge as it passes to said portal, said water particles being formed and propelled into said path by a dispersing eX- plosive charge disposed in water placed between said main explosive charge and said portal.

3. A process of claim 2 wherein said main explosive charge is disposed in a subterranean chamber and said water is dispersed along at least 10 feet of a tunnel running from said chamber to said portal, the weight of water dispersed beixig about from 10 to 100 times that of explosive in said main charge, and the ratio of the weight of water dispersed to the weight of explosive in said dispersing charge being about from 100/1 to 15,000/1.

4. A process of claim 3 wherein said portal is substantially closed by a massive plug.

5. A noise-reducing structure which comprises a stable subterranean chamber, a tunnel connecting said chamber to a portal, at least one body of water disposed in said tunnel and at least one explosive charge positioned in said Water to disperse said water in said tunnel.

6. A structure of claim 5 wherein said portal is P vided with a massive closure means.

7. A structure of claim 5 wherein said chamber and tunnel are in granite.

8. A structure of claim 5 wherein said chamber and tunnel are in limestone.

References Cited UNITED STATES PATENTS 232,640 9/1880 Hallock 10230 XR 3,222,872 12/1965 Langefors et al. 229-13 XR FOREIGN PATENTS 515,909 12/1952 Belgium. 520,488 6/1953 Belgium.

81,320 5/1895 Germany. 624,771 6/1949 Great Britain.

ROBERT S. WARD, JR., Primary Examiner. 

1. A PROCESS FOR REDUCING THE SHOCK AND NOISE CAUSED BY DETONATION OF A MAIN EXPLOSIVE CHARGE WHICH COMPRISES INTERPOSING A BODY OF DISPERSED WATER PARTICLES IN THE PATH OF THE SHOCK WAVE FROM SAID MAIN EXPLOSIVE CHARGE, SAID 